This invention relates to an electrostatic copying apparatus and more particularly to an electrostatic copying apparatus provided with a photosensitive layer whose light-irradiated portion is rendered conducting.
An electrostatic copying apparatus generally comprises a photosensitive drum which is formed of a drum body acting as a conductive support, and a photosensitive layer prepared from a photoconductive material and mounted on the outer peripheral wall of the drum body. The photoconductive material includes, for example, amorphous selenium, zinc oxide, cadmium sulfide, anthracene, polyvinyl carbazole, and those of the above-listed materials which are further mixed with a sensitizer. The electrostatic characteristic of the photosensitive layer varies with the kind of a raw photoconductive material or the method of manufacturing the raw photoconductive material.
The photosensitive layer is subjected to a copying cycle including electrification, exposure of an image to light, development, transcription, cleaning and discharge, all these steps being carried out in the order mentioned. An electrifying device electrifies the photosensitive layer to render it photosensitive. The electrification of the photosensitive layer by the electrifying device is carried out by exposing the photosensitive layer to a corona discharge and depositing the resultant ions on the surface of the photosensitive layer. The corona discharge is effected by impressing a voltage of several thousand volts on a space defined between a fine conductor stretched immediately above the photosensitive layer and the drum body. At this time, a corona emitted from the conductor ionizes the molecules of the surrounding air to produce ions. The photosensitive layer is electrified by the deposition of ions to have a predetermined surface potential.
The light source, for example a lamp, of the discharge device irradiates light beams and specified portion of the photosensitive layer for electric conduction. The discharge device releases electric energy charged on the specified portion by the following way. The electrons of a photoconductive material constituting the photosensitive layer are excited when absorbing light beams having a prescribed wavelength. A larger number of free carriers formed of free electrons and free holes are produced in the above-mentioned excitation than in the thermal equilibrium, rendering the photosensitive layer conducting. As a result, the photosensitive layer can no longer hold electric energy on the surface. In other words, electric energy passes along with the free carriers through the specified portion of the photosensitive layer which is now rendered conducting. Consequently, the photosensitive layer is discharged by being earthed through the drum body acting as a conductive support.
Where, however, the photosensitive layer is repeatedly electrified and discharged, then electric energy which is not completely released by the discharge is trapped or retained in the photosensitive layer. In other words, the so-called fatigue appears in the photosensitive layer. This means that as shown by curve (a) in FIG. 1, repeated copying operations give rise to an increase in the residual potential of the photosensitive layer due to the trapping of electric energy. Now let it be assumed one copying cycle takes 2.5 seconds with respect to a photosensitive layer whose photoconductive material is prepared from a seleniumtellurium alloy. Then, it has been experimentally found that trapped electric energy begins to be brought to a saturated condition at about the 100th copying cycle, the residual surface potential of the photosensitive layer reaching about 150 volts. As a result, a difference between the increased residual surface potential and development bias voltage as shown by curve (b) is prominently reduced as seen from curves (a) and (b) shown in FIG. 1. Thus, the fatigue of the photosensitive layer appears in the form of fogging on the white ground of an image transcribed on a copy sheet, resulting in a decline in the picture quality.
The trapped electric energy adversely affects the surface potential of the electrified photosensitive layer. In other words, the surface potential decreases, as seen from curve (c) shown in FIG. 1, from that which is produced when the copying operation is commenced by an amount corresponding to that of the trapped electric energy which is thermally released. Where a sample photosensitive layer was repeatedly electrified and discharged with the initial surface potential chosen to be 800 volts, then the initial surface potential of 800 volts fell to about 700 volts at about the 50th copying cycle as seen from FIG. 1 due to the saturated condition of the trapped electric energy. Accordingly, the fatigue of the photosensitive layer appeared in the form of a decline in the concentration of a transcribed image, that is, the picture quality.
Where, however, the deteriorated photosensitive layer is stored in the dark place during long time, then electric energy trapped in the photosensitive layer is gradually released by thermal energy resulting from the natural ambient temperature, thereby bringing the photosensitive layer to the original condition. Nevertheless, with an automatic electrostatic copying apparatus whose photosensitive layer is repeatedly electrified and discharged at a high speed, the photosensitive layer is continuously subject to fatigue. With a high speed electrostatic copying apparatus, wherein the photosensitive layer is normally allowed to have an extremely short rest time, the remaining fatigue of the photosensitive layer exerts a prominently adverse effect. In other words, the deterioration of the photosensitive layer presents great difficulties in ensuring the speedy and efficient operation of an electrostatic copying apparatus.
Hitherto, no decidedly effective resolution has been proposed with regard to the above-mentioned problems. Proposals of resolution advanced to date are intended only to control a voltage impressed on an electrifying device in accordance with changes in the surface potential of a photosensitive layer or an amount of light beams which are irradiated to provide a latent image. An electrostatic copying apparatus set forth in, for example, the recent Japanese patent disclosure No. 148,444 (1978) is intended to effect improvements on the copying process. The object of this patent disclosure is to irradiate light beams on a photosensitive layer immediately before its electrification particularly to suppress changes in the static characteristic of the photosensitive layer when repeatedly electrified and discharged at a high speed. With the above-mentioned patent disclosure, a decline in the picture quality of an image transcribed on a copy sheet is indeed reduced to an appreciably satisfactory extent. But an electrostatic copying apparatus embodying that patent disclosure would extremely increase in cost if practically constructed. Proposals advanced to date regarding an electrostatic copying apparatus may be considered to have achieved a noteworthy improvement, if cost is neglected. Yet viewed from a fundamental patent object of making a noteworthy contribution to industrial development, any new technique, if involving a high manufacturing cost, can hardly be regarded as a truthful improvement on the prior art.